1. Field of the Invention
The present invention relates to a solid-state image-sensing device, and particularly to a solid-state image-sensing device having pixels arranged in a two-dimensional array.
2. Description of the Prior Art
Conventionally, a solid-state image-sensing device having photosensitive elements such as photodiodes (hereinafter, such an image-sensing device will be referred to as an xe2x80x9carea sensorxe2x80x9d) has constant-current sources, for amplifying the output signals from the individual pixels, provided one for each column of pixels. An example of such an area sensor is shown in FIG. 15.
In FIG. 15, reference symbols G11 to Gmn represent pixels arranged in rows and columns (in a matrix). Reference numeral 51 represents a vertical scanning circuit, which scans rows (lines) 53-1, 53-2, . . . , 53-n sequentially. Reference numeral 52 represents a horizontal scanning circuit, which reads out photoelectric conversion signals, which are delivered from the pixels to output signal lines 55-1, 55-2, . . . , 55-m, sequentially pixel by pixel in the horizontal direction. Reference numeral 54 represents a power line.
Moreover, as shown in the figure, for each of the output signal lines 55-1, 55-2, . . . , 55-m, a pair of N-channel MOS transistors Q1 and Q2 is provided. Here, a description will be given only with respect to the output signal line 55-1 as a representative. The MOS transistor Q1 has its gate connected to a direct-current voltage line 56, has its drain connected to the output signal line 55-1, and has its source connected to a line 57 of a direct-current voltage VPSA. On the other hand, the MOS transistor Q2 has its drain connected to the output signal line 55-1, has its source connected to a signal line 58 serving as a final destination line, and has its gate connected to the horizontal scanning circuit 52.
In the pixels G11 to Gmn provided in the area sensor shown in FIG. 15, photoelectric charges occur, and the pixels G11 to Gmn feed output currents based on those photoelectric charges to the output signal lines 55-1 to 55-m. Since the MOS transistors Q1 have their drains connected to the output signal lines 55-1 to 55-m and receive a direct-current voltage DC at their gates all the time, they are equivalent to resistors or constant-current sources, and thus serve to voltage-amplify the output currents fed to the output signal lines.
The MOS transistors Q2 are controlled by the horizontal scanning circuit 52 so as to act as switching devices for selecting a column. With the MOS transistors Q1 configured in this way, it is possible to yield output signals at a high gain. As a result, even if the output signals from the individual pixels are low, the amplifier circuit including those MOS transistors Q1 amplifies them to a sufficiently high level, making them easier to process in the signal processing circuit (not shown) in the succeeding stage.
However, the circuit configuration described above has the following disadvantage. As shown in FIG. 15, the output signals from the individual pixels are amplified by the MOS transistors Q1 provided one for each column. If the characteristics of these MOS transistors Q1 vary among them, the output signals from the pixels belonging to different columns are amplified by different amplification factors. Accordingly, although there is no variation among the output signals from the pixels arranged in an identical column, there appear variations among the output signals from the pixels arranged in an identical row because those output signals are amplified by different amplification factors. As a result, when an image is reproduced from the output signals obtained from an area sensor like this, variations in the amplification factors among the MOS transistors Q1 that are provided, one for each column, cause fixed pattern noise that looks like vertical stripes.
An object of the present invention is to provide a solid-state image-sensing device capable of canceling fixed pattern noise resulting from variations in the output signals of the solid-state image-sensing device due to the circuit configuration thereof or the like.
To achieve the above object, according to one aspect of the present invention, a solid-state image-sensing device is provided with: a first pixel including a photoelectric conversion element and capable of generating an output signal that is logarithmically proportional to the amount of light incident on the photoelectric conversion element; a second pixel for generating as an output signal a compensation signal with which to compensate the output signal of the first pixel; and a reading circuit for reading out the output signals of the first and second pixels.
According to another aspect of the present invention, a solid-state image-sensing device is provided with: a first pixel including a photoelectric conversion element and capable of generating selectively either an output signal that is logarithmically proportional to the amount of light incident on the photoelectric conversion element or an output signal that is linearly proportional to the amount of light incident on the photoelectric conversion element; a second pixel for generating as an output signal a compensation signal with which to compensate the output signal of the first pixel; and a reading circuit for reading out the output signals of the first and second pixels.